1. Field of the Invention
The present invention relates to a double-side illumination apparatus for a display device and a dual display device employing the same and, more particularly, to a double-side illumination apparatus which can provide light having a specific polarization component through upper and lower surfaces, respectively, and a dual display device employing the same.
2. Description of the Related Art
In recent years, with the rapid spread of folder type mobile phones having a dual display, the need for effectively supplying light to two liquid crystal display panels has increased. In the dual display, two illumination apparatuses are used for providing light to the two liquid crystal display panels, respectively. However, this leads to increased power consumption and a limitation on how thin the liquid display panel can be. Accordingly, a technology for a double-side illumination apparatus is being developed, in which light can be simultaneously provided to two liquid crystal panels using one illumination apparatus.
FIG. 1 is a sectional view illustrating an example of a related art double-side illumination apparatus 10 and a dual display device employing the same. In the double-side illumination apparatus 10 of FIG. 1, a light emitted from a light source 11 is incident into a light guide plate 12 through one side of the light guide plate 12. Prism patterns 13 and 14 are formed on both side surfaces of the light guide plate 12. Therefore, a portion of light propagating inside the light guide plate 12 is totally reflected by the lower prism pattern 14 and out-coupled toward an upper surface of the light guide plate 12. Another portion of light propagating inside the light guide plate 12 is totally reflected by the upper prism pattern 13 and out-coupled toward the lower surface of the light guide plate 12. As illustrated in FIG. 1, upper and lower liquid crystal display panels 15 and 16 are disposed above and below the light guide plate 12, respectively. Accordingly, the light out-coupled through the upper surface of the light guide plate 12 is incident on the upper liquid crystal display panel 15, and the light out-coupled through the lower surface of the light guide plate 12 is incident on the lower liquid crystal display panel 16.
Since the light out-coupled through the upper and lower surfaces of the light guide plate 12 in the double-side illumination apparatus 10 of FIG. 1 is unpolarized light, 50% of the light is absorbed by absorption polarizers included in the upper and lower liquid crystal display panels 15 and 16. As a result, optical efficiency of the double-side illumination apparatus 10 is degraded.
FIG. 2 is a sectional view illustrating another example of a related art double-side illumination apparatus 20 and a dual display device employing the same. The double-side illumination apparatus 20 of FIG. 2 includes a light guide plate 21 having a prism pattern 25 only on a lower surface thereof for totally reflecting incident light, a light source 22 disposed at one side of the light guide plate 21, and a reflective polarizer 24 disposed above the light guide plate 21. In such a configuration, the light emitted from the light source 22 is incident on a side surface of the light guide plate 21 and propagates inside the light guide plate 21. The light propagating inside the light guide plate 21 is totally reflected by the prism pattern 25 on the lower surface of the light guide plate 21 and then out-coupled toward the upper surface of the light guide plate 21. Among the light out-coupled toward the top surface of the light guide plate 21, light with the first polarization component is transmitted through the reflective polarizer 24, whereas light with the second polarization component, which is orthogonal to the first polarization, is reflected toward the light guide plate 21. Accordingly, a portion of the light is incident on an upper liquid crystal panel 26 disposed above the reflective polarizer 24, and another portion is incident on a lower liquid crystal display panel 27 disposed below the light guide plate 21.
In the double-side illumination apparatus 20 of FIG. 2, the light incident on the upper liquid crystal display panel 26 has only a specific polarization component so that loss does not occur. However, because the light incident on the lower liquid crystal display panel 27 is re-transmitted through the light guide plate 21, reflection loss occurs and further the polarization is not preserved while the light passes through the light guide plate 21. Therefore, the amount of light incident on the lower liquid crystal display panel 27 is reduced due to the reflection loss, and a portion of the light may be absorbed by the absorption polarizer included in the lower liquid crystal display panel 27 as well. As a result, brightness is more degraded in the lower liquid crystal display panel 27 than in the upper liquid crystal display panel 26. Moreover, because the reflective polarizer 24, which is used for operation in a wide-angle range, is expensive, the total price of the dual display device is increased.